The recent trend in unsupervised hashing requires not only a discrete representation space but also the ability to mine the similarities between data points. Determining and maintaining the relations between each datum and all the others maximally utilize the semantic diversity of the training set, but can we explore this on the holistic dataset using a single network end-to-end? In this paper, we take a step towards this vision by proposing Overview Hashing (OH). OH unifies the two ultimate goals of unsupervised hashing, i.e., (1) encoding compact features and (2) learning data similarities on a large scale end-to-end, into one model. In particular, we split the top of an encoder into a binary hash head and a continuous one. For an arbitrary datum, its similarities to all the others in the dataset are reflected in the Hamming distances of their hash heads. The distances then act as the weights to aggregate the continuous heads, shaping the final representation of this datum for loss computation. Hence, training with this representation simultaneously tunes the similarities of this datum to the whole dataset. In the context of a contrastive learning framework, we theoretically endorse our design by linking it to knowledge distillation and the attention mechanisms. Our experiments on the benchmarked datasets show the superiority of OH over the state-of-the-art hashing methods. Code is available at \hrefhttps://github.com/RosieYuu/OH \textcolorred https://github.com/RosieYuu/OH.

Learning to hash pictures a list-wise sorting problem. Its testing metrics, e.g., mean-average precision, count on a sorted candidate list ordered by pair-wise code similarity. However, scarcely does one train a deep hashing model with the sorted results end-to-end because of the non-differentiable nature of the sorting operation. This inconsistency in the objectives of training and test may lead to sub-optimal performance since the training loss often fails to reflect the actual retrieval metric. In this paper, we tackle this problem by introducing Naturally-Sorted Hashing (NSH). We sort the Hamming distances of samples' hash codes and accordingly gather their latent representations for self-supervised training. Thanks to the recent advances in differentiable sorting approximations, the hash head receives gradients from the sorter so that the hash encoder can be optimized along with the training procedure. Additionally, we describe a novel Sorted Noise-Contrastive Estimation (SortedNCE) loss that selectively picks positive and negative samples for contrastive learning, which allows NSH to mine data semantic relations during training in an unsupervised manner. Our extensive experiments show the proposed NSH model significantly outperforms the existing unsupervised hashing methods on three benchmarked datasets.